What is the True Solubility Advantage for Amorphous Pharmaceuticals?
- Cite this article as:
- Hancock, B.C. & Parks, M. Pharm Res (2000) 17: 397. doi:10.1023/A:1007516718048
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Purpose. To evaluate the magnitude of the solubility advantage foramorphous pharmaceutical materials when compared to their crystallinecounterparts.Methods. The thermal properties of several drugs in their amorphousand crystalline states were determined using differential scanningcalorimetry. From these properties the solubility advantage for theamorphous form was predicted as a function of temperature using a simplethermodynamic analysis. These predictions were compared to theresults of experimental measurements of the aqueous solubilities of theamorphous and crystalline forms of the drugs at several temperatures.Results. By treating each amorphous drug as either an equilibriumsupercooled liquid or a pseudo-equilibrium glass, the solubilityadvantage compared to the most stable crystalline form was predicted to bebetween 10 and 1600 fold. The measured solubility advantage wasusually considerably less than this, and for one compound studied indetail its temperature dependence was also less than predicted. It wascalculated that even for partially amorphous materials the apparentsolubility enhancement (theoretical or measured) is likely to influencein-vitro and in-vivo dissolution behavior.Conclusions. Amorphous pharmaceuticals are markedly more solublethan their crystalline counterparts, however, their experimental solubility advantage is typically less than that predicted from simplethermodynamic considerations. This appears to be the result of difficulties indetermining the solubility of amorphous materials under trueequilibrium conditions. Simple thermodynamic predictions can provide a useful indication of the theoretical maximum solubility advantage foramorphous pharmaceuticals, which directly reflects the driving forcefor their initial dissolution.